The aggregation behavior of polybutadiene(165)-block-poly(L-lysine)(88) in saline solution was studied by combined static and dynamic light scattering analyses. Vesicles were observed if the polypeptide segment was in a 100% coil conformation (pH 7.0) or in an 80% alpha-helical conformation (pH 10.3). At the higher pH, aggregates were smaller in size (hydrodynamic radius: 364 nm -> 215 nm) and chains were more densely packed at the core-corona interface (interchain distance: 3.2 nm -> 2.4 nm). Changes in size and structure could be explained in basic terms of colloid stabilization without considering a secondary structure effect.